Given a data structure with posts that have an author, I understand that a request for /posts/123/author might return the user resource that is related to post 123 as being the author.
However, what is the correct response when a non-relationship field is requested? For example, /posts/123/title (title being the title of the post; a simple string).
Should it be answered with a 400 status code? Should it be treated like a sparse fieldset request?